Sediment Transport
Significant sediment transport occurs along the island chain over the course of the Delft3d simulation. Mean total transport magnitudes appear to be a function of depth, and are highest at the shallowest locations (Figure 19). The highest magnitudes of transport of are observed along of Chandeleur Island, but significant transport also occurs near Breton Island as well as Curlew, Grand Gosier, and Gosier Shoals. Transport near these southern shoals increases with both shoal size and proximity to Chandeleur Island. Curlew
experiences the most transport, followed by Grand Gosier, and then Gosier Shoals. As the profile is raised, transport magnitudes decrease near Chandeleur Island and increase near the southern shoals. Higher morfac runs experience decreased transport near Chandeleur Island and the southern shoals.
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Figure 19: Mean total transport magnitudes for the entire island chain for initial and raised bed levels. Previous work suggests that a seasonally fluctuating nodal point in sediment transport direction exists near the middle of Chandeleur Island (Ellis and Stone, 2006; Georgiou and Schindler, 2009). During the cold front season, this point shifts slightly north of the center of the island. Our results confirm the existence of this point, and, as expected for a winter storm, place it near the cold front season location. Figure 18 shows normalized mean sediment transport for the entire chain.
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North of the nodal point the dominant longshore transport direction is north, while sediment south of the nodal point moves south.
A previous energy balance for the system determined that energy input into the sound is strongest to the north of the chain. The southern end experienced equal energy gain and loss, while inlets experienced energy loss (Hart and Murray 1978). Our storm produces this same general flow pattern, and it is evident in plots of mean total transport vectors along the chain (Figure 20). There is a noticeable flood dominant transport over the spit platform to the north of Chandeleur Island. Over most of the southern shoals, transport is generally cross-shore balanced and primarily southward. Transport is ebb dominant in southern inlets. Additionally, since water is predominantly entering the basin to the north of the chain and exiting to the south, there is consistent southward transport behind the barrier.
Figure 20: Normalized mean total transport vectors showing divergence of longshore transport opposite nodal point (blue line) for the unraised base-case scenario.
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These sediment transport patterns due to longshore transport and backbarrier circulation induce significant sedimentation and erosion for the southern shoals. Results are presented in figures 21-23. Higher morfac runs exhibit similar sedimentation and erosion patterns; however, the magnitudes of both sedimentation and erosion are
amplified. Curlew Shoal exhibits patterns typical of landward migration. The seaward sides of the shallowest depths of the shoal experience erosion, while the central and landward sides experience sedimentation. Gosier and Grand Gosier experience patterns typical of spit growth and elongation. The center of each shoal experiences erosion, while the northern and southern edges experience aggradation. Raising bed levels increases magnitudes and changes sedimentation and erosion patterns for Curlew Shoal. Raised scenarios experience increased landward migration. Raising bed levels for Gosier and Grand Gosier Shoals increases magnitudes of change, but does not appear to have a significant impact on sedimentation and erosion patterns. All scenarios experience erosion in inlets and deposition in ebb tidal deltas. Scenario 1 has the largest spatial extent of erosion and the smallest magnitudes of sedimentation. Scenario 3 has the largest magnitudes of both sedimentation and erosion.
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Figure 21: Sedimentation and erosion patterns for the unaltered scenario at initial and raised bed levels.
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Figure 22: Sedimentation and erosion patterns for each of the four base scenarios with unraised bed levels.
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Figure 23: Sedimentation and erosion patterns for each of the four base scenarios with bed levels raised by 1m.
36 Shoal Aggradation
Since aggradation and degradation also depend on initial depths, plan views of sedimentation and erosion do not accurately depict aggradation or degradation of the shoal. Shoal aggradation can be masked by shoal widening and landward migration of the entire shoal. Figures 24 and 25 depict aggradation and degradation of the shoal crest using an alongshore transect of the shoal crest elevation from south to north. Aggradation and degradation trends indicate southern movement of sediment along the chain. This is evident in the downdrift movement of shoals and inlets, in agreement with previous observations of sediment transport in this region (Georgiou and Schindler 2009), and is also apparent in our mean total sediment transport results. Slight degradation is seen along most of the transect; Gosier experiences significant destructive reworking, and shoal crests lower approximately 1m in every scenario. Grand Gosier experiences slight aggradation in morfac 1 simulations, and degradation in morfac 20 simulations. This degradation weakens as the transect is raised. Curlew Shoal, on the other hand, experiences significant
aggradation in raised scenarios. Unraised scenarios experience minimal aggradation or degradation. Scenario 1 experiences slight degradation, while scenarios 2-4 experience slight aggradation. As the bathymetry is raised, aggradation increases, and the differences between scenarios becomes more apparent. Scenario 4 experiences the most aggradation, while scenario 1 experiences the least.
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Figure 24: Alongshore crest elevations for unraised and raised base scenario simulations using morfac values of 1 (left) and 20 (right). The top plot shows results for all unraised and raised scenarios with bed levels normalized to the unraised state. The lower plots show initial and final shoal depth for each initial bed level. The transect starts near the MRGO channel (left) and extends to north of Curlew Shoal (right). “GG” represents Grand Gosier and “G” represents Gosier. Scenarios are
named xpy, where x is the base scenario, and y is the amount that bed levels have been raised.
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Figure 25: Alongshore crest elevations for unraised (top) and raised scenarios 1-4 using morfac values of 1 (left) and 20 (right). Mean sea level is plotted as a horizontal blue line. The transect starts near the MRGO channel (left) and extends to north of Curlew Shoal (right). Results from raised
scenarios have been normalized to initial bed levels. Scenarios are named xpy, where x is the base
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